Before Mark Zuckerberg testified in front of Congress last year, he and his team prepared by holding mock hearings inside a Facebook conference room set up to look like a congressional room.

For Zuckerberg, the appearance marked his first time testifying before Congress, a forum that had eaten other prominent technology executives alive. For Facebook, it came at a crucial moment in the wake of the Cambridge Analytica data scandal, which caused Facebook to shed tens of billions in market value and sparked fierce criticism from users and politicians around the world.

"That was as high stakes as it gets," Sheryl Sandberg, Facebook's COO, recalled in an interview last year for a forthcoming CNN documentary called "Facebook at 15: It's Complicated." "Mark prepared so carefully."

Yet, Sandberg, Zuckerberg's longtime colleague and confidant, refrained from attending all of his prep sessions. The reason, as she told CNN Business' Laurie Segall in the interview, offers a glimpse into their working relationship. "Mark and I try hard to stay focused and divide and conquer forever," she said.

For years, the Silicon Valley power duo thrived by doing just that. Zuckerberg focused on Facebook the product, while also pushing for billion-dollar acquisitions like Instagram and WhatsApp. Sandberg, meanwhile, focused on Facebook the business, building up and overseeing the social network's massive ad-sales machine.

The divide and conquer strategy proved successful. In 2008, the year Zuckerberg hired Sandberg away from Google, Facebook generated $272 million in revenue, but failed to turn a profit. Ten years later, Facebook's annual revenue topped $55 billion -- and it posted a $22 billion profit.

The pair helped to firmly establish Facebook as an internet behemoth with vast influence over the media and advertising worlds, a trail of vanquished rivals and a market valuation hovering around $500 billion.

As Facebook flourished, so did their personal brands. At various points in recent years, both executives have been floated as possible presidential candidates in the press. Zuckerberg met with world leaders and started a book club; Sandberg wrote bestselling books -- first about women in the workplace and then, after losing her husband, on grief.

The pair were so close that Zuckerberg took a lead role in planning the funeral for Sandberg's husband, Dave Goldberg, the former CEO of SurveyMonkey. The Facebook founder's involvement didn't stop there. Zuckerberg helped Sandberg deal with the anxiety of returning to work and even invited Sandberg and her kids to join them on a vacation a few months after Goldberg's death.

"Mark talked to me every day, came over to my house every day and was just there for me and my children -- and his wife, Priscilla was -- in every way possible," Sandberg recalled in the interview.

But these two executives, who more than anyone else are the faces of Facebook, have been put to the ultimate test since the 2016 presidential election. The company has been hit with a endless series of crises over data privacy, the spread of fake news and foreign election meddling.

As Zuckerberg admitted on a conference call with analysts in January, "a lot of our business challenges have been self-imposed." That list likely includes a PR scandal late last year over a New York Times investigation that found Facebook had attempted to ignore and conceal Russian interference on its platform. The Times also reported that Facebook had hired a public relations firm that dug up dirt on its competitors and circulated information on billionaire George Soros, which attempted to link him to groups pushing for more regulation of Facebook.

The damning report led to renewed speculation about whether changes might finally come to Facebook's leadership. Some investors called for Zuckerberg to step down as the chairman of Facebook's board of directors. He refused.

The steady drip of scandals also damaged Sandberg's brand, inside and outside Facebook. Multiple reports said Sandberg was being blamed more within Facebook for its troubles, raising doubts about her future at the company.

Sandberg, who was interviewed for the documentary before the Times investigation, told CNN Business that she's made safety and security efforts "a much bigger focus of my time" as scrutiny of Facebook has mounted.

"We've realized we missed stuff. And we don't want to make that mistake again," she said.

Alex Stamos, the former chief security officer at Facebook, said in the CNN documentaryinterview that Facebook's problems were not primarily Sandberg's fault so much as the result of "thousands of product decisions that were made, not considering the actual threat that social media can play to democracies, and not thinking about the adversarial use of the product."

"Those product decisions were not Sheryl's responsibility," Stamos said in an interview for the documentary.

For people "to try to shift responsibility onto one of the most powerful women in Silicon Valley, I'm honestly a little upset," he said.

Zuckerberg, who has previously suggested the buck stops with him for Facebook's failings, defended Sandberg in an interview in November, citing their long and lucrative partnership.

"Sheryl is a really important part of this company and is leading a lot of the efforts for a lot of the biggest issues that we have," he said. "She's been an important partner to me for ten years. I'm really proud of the work that we've done together and I hope that we work together for decades more to come."